The present disclosure relates generally to power supplies, and more particularly to apparatuses and control methods for providing dummy loads to power supplies under primary-side control.
The battery run time, the duration when a portable device is operable under the power supplied by its own batteries, means a lot to users. A short battery run time troubles user in non-operable device or frequently charging. To make battery run time longer, the capacity of the batteries in portable devices becomes larger. Aside effect of the batteries with larger capacity is a longer charging time which is required for a battery charger to charge the batteries to a full condition. Some manufactories of battery chargers have developed methods for quickly charging batteries, so users need not wait so long to fully charge their portable devices.
A common methodology of quickly charging batteries is to increase the output voltage supplied by a battery charger. For example, a USB port of a charger has an output rating voltage about 5V, but that charger, if equipped with the ability of quickly charging, might boost its output voltage up to between 9V and 12V to charge a portable device. Nevertheless, a portable device that is to be charged by a 9V input voltage must be specially designed to sustain a so-high charge voltage. Otherwise, that portable device could be over stressed and suffer damage.
To be backward compatible with old-version portable devices that are unable to sustain a high-charge voltage, the output voltage of a battery charger with the ability of quickly charging must lower its output voltage down to its output rating voltage quickly after a portable device is removed, so that the charger won't damage any of old-version portable devices that does not support quickly charging and is next connected to the output port of the charger.
Demonstrated in FIG. 1 is a conventional charger 100 with the ability of quickly charging, for charging the load 104. The charger 100 has an isolation topology, with a primary side and a secondary side isolated from each other by a transformer. Voltages at the primary side substantially reference to input ground GNDIN, while voltages at the secondary side to output ground GNDOUT. As illustrated in FIG. 1, a power controller 108 in the primary side turns ON and OFF a power switch 106 so as to control the current through a primary winding PRM. When power switch 106 is turned ON, the current through the primary winding PRM increases and the transformer energizes; when it is turned OFF, the transformer de-energizes and the secondary winding SEC outputs a current to build output voltage VOUT. The auxiliary winding AUX, the secondary winding SEC, and voltage divider 110 cooperate to provide information in association with the output voltage VOUT, and power controller 108 accordingly provides pulse-width-modulation (PWM) signal SDRV to control power switch 106. This type of control is commonly referred to as primary side control (PSR), which detects output voltage VOUT by way of the induced voltage of a transformer, rather than through a photo-coupler. The charge 100 includes a dummy load RDUM, which is capable of lowering the output voltage VOUT down to a safe level when the load 104 is removed or becomes a light load.
The presence of the dummy load RDUM causes disadvantages in view of power conversion, because it constantly consumes electric power no matter the load 104 exists or not. Therefore, the dummy load RDUM is not suitable for advanced chargers, especially for those seeking a higher power conversion rate.